1. Field of the Invention
The present invention generally relates to a method for crystallizing amorphous layer. More particularly, the present invention relates to a method for crystallizing amorphous layer wherein the amorphous layer is coated with a nickel-containing solution and heated afterwards.
2. Discussion of Related Art
Polysilicon layer has been employed for the active layer of a thin film transistor in semiconductor devices, notably liquid crystal display devices. This is because the polysilicon layer has a high carrier mobility relative to amorphous silicon layer. Such polysilicon is normally grown at high temperatures. However, there have been recently developed techniques for fabrication of polysilicon thin film transistors at low temperatures.
This low-temperature polysilicon involves some advantages in that it can be processed at low temperatures, can form large areas and can perform comparably to high formation-temperature polysilicon.
Many techniques are known for crystallization of amorphous silicon layer, such as SPC (Solid Phase Crystallization), laser crystallization and so forth. Laser crystallization is a technique wherein amorphous silicon layer is subjected to a heat treatment making use of a laser for crystallization at low temperatures, i.e. less than 400.degree. C. (see Hiroyaki Kuriyama et al., Jpn, J. Phys. 31, 4550 (1992)). Laser crystallization, however, is not useful for the purpose of formation of polysilicon layer on a substrate which is large in area, because the polysilicon layer in this case is not formed uniformly and costs a great deal due to the use of expensive equipment for which the yields are low.
In the SPC technique, amorphous silicon layer is heated in the 550.degree. to 700.degree. C. range of temperature for about 1-24 hours. This has the advantage that the amorphous silicon layer crystallizes uniformly with inexpensive equipment. However, it is inapplicable to a glass substrate since the layer crystallizes at relatively high temperatures only after a long time, plus the yields are low.
Metal-induced crystallization is another example of a method for crystallizing amorphous silicon at low temperatures (See S. Haquc et al., Appl. Phys. 79, 7529 (1996)). In metal-induced crystallization, amorphous silicon is brought into contact with a specified kind of metal in order to lower the crystallization temperature. For example, in a nickel-induced crystallization, the final nickel silicide phase, NiSi.sub.2 acts as a crystal seed that promotes polycrystalline silicon growth (see C. Hayzelden et al., J. Appl. Phys. 73, 8279 (1993)). Nickel silicide NiSi.sub.2 has a silicon-like structure with a lattice constant of 5.405 .ANG., approximate to that of silicon (5.340 .ANG.), and accelerates the change of amorphous silicon into poly silicon (See C. Hayzelden et al., Appl. Phys. Lett. 60, 225 (1992)). When metal-induced crystallization is applied to a prior art, metal solid layer having a certain thickness is deposited on amorphous silicon layer by sputtering technique. And then, the amorphous silicon layer having metal solid layer thereon is subjected to a heat treatment.
The period of time and temperature of the heat treatment as well as the amount of metal used can affect metal-induced crystallization. For example, crystallization temperature becomes lowered with an increase in the amount of metal. For the metal-induced crystallization described above, amorphous layer crystallizes at low temperatures with growth efficiency increasing in proportion to the amount of metal. Metal functions as catalyst for crystallizing the amorphous layer.
However, an example of the problem with such a metal-induced crystallization of amorphous silicon lies in the change of inherent properties of the silicon layer that results from contaminant metals entering the polysilicon layer. Furthermore, heat treatment takes a long time i.e. 10 hours or more, and the growth temperature is relatively high.
Despite the metal-induced crystallization being attainable at low temperatures, the natural properties of silicon layer may be changed by the presence of contaminating metals. An increase in the amount of metal substantially increases the efficiency of metal-induced crystallization, but also raises the problem of metal-contamination. It is therefore desirable to reduce metal-contamination of silicon layer due to the metal-induced crystallization as well as to lower the crystallization temperature.